Management of broomrape (Orobanche cernua) in tobacco (Nicotiana tabacum)

Tobacco is an important commercial crop in India. India is the third largest tobacco producing country in the world. Tobacco is cultivated in an area of 0.428 million ha. Non- Virginia tobaccos such as bidi tobacco constitute about 65% of the total tobacco area in the country.

Broomrape (Orobanche cernua) is a debilitating holoparasitic weed in all tobacco growing areas in India, with a devastating effect on the crop. In India, yield loss in tobacco ranges from 30 - 70%; at present hand weeding is the only practice in India applied to control the parasite.

With this background, several field and laboratory experiments were conducted in Karnataka State, Southern India, to study the germination biology and to develop a suitable method to induce the germination of the parasite, and to develop a technology by integrating agronomic and chemical approaches to control the parasite at different phases.

The germination phase of the parasite is a critical period. The seed bank of the parasite cae be reduced by stimulating the germination through chemicals, natural host stimulants or both. GR24 (a strigol analogue) at 1.0 and 0.1 ppm, was the standard to assess potential germination. Of the other chemicals, gibberellic acid at 10 and 20 ppm was most effective. The stimulating effects of host plants were significant even when GR24 was applied. Suicidal germination of the parasitic seeds triggered by growing trap crops reduced the weed population and the growth of the host plants was hastened due to green manuring effect of trap crops. Therefore, including a trap crop in the rotation may reduce the problem. Sunhemp (Crotalaria juncea L.) and greengram (Vigna radiata L.) are promising trap crops in a cropping system containing bidi tobacco in areas where tobacco is grown in a long growing season.

Chemical control by (systemic) herbicides is also an option. Maleic hydrazide (MH) reduced broomrape spikes at 0.25 - 0.75 kg a.i./ha applied at 30 or 40 days after transplanting (DAT) tobacco. Higher tobacco yields were obtained with 0.25 kg a.i./ha MH, which was on par with the hand weeding treatment both in "infested" and "non- infested" tobacco plants. Higher concentrations of MH were toxic to tobacco crop. Glyphosate at 0.50 kg a.i./ha applied at 60 DAT and imazaquin at 0.01 kg a.i./ha applied at 30 DAT reduced the broomrape population by almost 80% and increased tobacco leaf dry weight by more than 40% compared to the control treatment. Imazapyr and EPTC were less effective.

Swabbing natural plant oils killed the bud and stem parts of the parasite by suffocation. Neem, coconut and sunflower oils showed quick knock-down effects in killing the bud part, whereas neem oil did not kill the stem part of the parasite. Niger, castor and mustard oils appeared to be (somewhat) less effective.

In general, there is a negative linear relation between broomrape infestation and tobacco yield, with a very large (negative) regression coefficient.

No single method is effective in controlling the parasite. The seed bank of the parasite should be minimized in a phased manner by integrating cultural and chemical methods of control. Therefore, an integrated management strategy is the best perspective to control broomrapes in a crop wherever it is problematic.

Key words: bidi tobacco, broomrape, chemical control, Crotalaria juncea, gibberellic acid, germination stimulants, GR24, herbicides, integrated weed control, natural plant oils, Orobanche cernua, parasitic weed, suicidal germination, trap crop, Vigna radiata.